The present invention generally relates to character recognition systems which recognize, employing a computer or the like, various image information such as hand- or machine-printed characters, marks, numerals and special symbols (hereafter represented by characters for simplicity). More particularly, the present invention relates to a method of assigning direction codes to boundary picture elements which is applicable to such a system to extract a given group of picture elements as strokes from a character pattern which is in the form of a congregation of two-level picture element data.
Various methods have heretofore been proposed for the recognition of characters. In one of such methods, a given group of picture elements are extracted as strokes out of a character pattern or congregation of two-level picture element data and the resulting strokes are used as characteristic parameter for character recognition. This method may be carried out by detecting end points and intersecting points of lines contained in a character, extracting as strokes the segments between the intersecting points, those between the intersecting points and the end points and those between the end points, and recognizing the character on the basis of the respective strokes.
Meanwhile, there have also been proposed various methods for so extracting strokes from character patterns. In one method which appears particularly effective, attention is given to a certain boundary picture element contained in a character pattern, that is, a black picture element which forms the character together with numerous other black picture elements and is neighbored by a white picture element in the up-down or right-left direction. The direction of connectivity of the specific black picture element on the character pattern is identified to assign to the boundary picture element (black picture element) a direction code corresponding to the specific direction of connectivity. Such a procedure is repeated for each of all the boundary picture elements. Then, the character pattern is scanned in different directions for the respective direction codes in order to check the connectivity of the direction codes, so that the congregation of picture elements with interconnecting common direction codes is extracted as a stroke.
A problem has existed in such a prior art method in that, when direction codes are assigned to boundary picture elements of a two-level image provided by sampling, for example, a slanted or curved line on a character pattern, different direction codes appear in undesirable zig-zag fashion at every other or more picture elements.